LWIP_NETCONN_SEM_PER_THREAD macro

LWIP_NETCONN_SEM_PER_THREAD==1: Use one (thread-local) semaphore per thread calling socket/netconn functions instead of allocating one semaphore per netconn (and per select etc.) ATTENTION: a thread-local semaphore for API calls is needed: - LWIP_NETCONN_THREAD_SEM_GET() returning a sys_sem_t* - LWIP_NETCONN_THREAD_SEM_ALLOC() creating the semaphore - LWIP_NETCONN_THREAD_SEM_FREE() freeing the semaphore The latter 2 can be invoked up by calling netconn_thread_init()/netconn_thread_cleanup(). Ports may call these for threads created with sys_thread_new().